Field of the Invention
The present invention relates to a servo control device for controlling a shaft of an arm or the like of a machine tool or a robot, and in particular to tandem control for controlling one control target, using two motors.
Description of the Related Art
In a drive mechanism of a machine tool, a robot, or the like, when a control target, or a moving unit, is so large that a torque (a thrust force in the case of a linear motion type) of a single motor for driving a shaft of the moving unit is insufficient, tandem control may be executed by giving a command to two motors to drive a single control target, using two motors. In the tandem control state, respective motors (a rotation type or a linear type) drive one control target in a rotation direction or a linear motion direction via a gear or a coupling element.
FIG. 8 is a schematic diagram showing a target plant in the form of a rotating motor shaft in the tandem control state. Note here that an inertia moment of a drive shaft 1 and a drive shaft 2 including two motors and a control target is expressed as being divided into I1 and I2, and deflection caused between the drive shaft 1 and the drive shaft 2 is expressed by a spring system with rigidity K. The respective positions of the drive shaft 1 and the drive shaft 2 are indicated by x1, x2, a disturbance torque is denoted as τdis, and respective generated torques as τ1, τ2.
An equation of motion in the case of driving the target plant shown in FIG. 8 under tandem control is expressed as expressions (1) and (2).[Expression 1]I1{dot over (v)}1+Σr=τdis  (1)I2{dot over (v)}2−τr=τ2+τdis  (2)
wherein the respective velocities of the drive shaft 1 and the drive shaft 2 are denoted as v1, v2, and a deflection torque τr is expressed as an expression (3).[Expression 2]τr=K(x1−x2)  (3)
FIG. 9 is a block diagram showing an equation of motion of a target plant in the tandem control state.
FIG. 10 is one example of a block diagram of a conventional position control device 300 for controlling the positions x1, x2 of a target plant in the tandem control state according to a position command value X generated as a function for every constant period by an upper-level device (not shown). A first shaft position control unit 100a controls the drive shaft 1, and a second shaft position control unit 100b controls the drive shaft 2. In the description below, for each of the members having the same function each provided for every shaft, either a (first shaft) or b (second shaft) is added to the end of the reference numeral thereof. In the following, an operation of the conventional position control device 300 shown in FIG. 10 will be described.
Initially, the first shaft position control unit 100a will be described. According to the conventional art, a feedforward structure is employed in order to achieve high speed response to a command. Specifically, an acceleration/deceleration processing unit 50a executes acceleration/deceleration processing for appropriate acceleration or derivative of acceleration with respect to a position command value X to output a position command value Xc subjected to such acceleration/deceleration processing. The position command value Xc is subjected to time differentiation in a differentiator Ma thereby giving a velocity feedforward amount VF, and further subjected to time differentiation in the differentiator 55a thereby giving an acceleration amount command value AF. An amplification rate ATF1 of an amplifier ATF1 is a constant for obtaining an acceleration/deceleration torque feedforward amount τF1 corresponding to a motor torque that accelerates the target plant 200 shown in FIG. 8 by an acceleration amount AF.
The feedback structure is formed as described below. That is, using the position x1 of the drive shaft 1, detected by a position detector (not shown), as a position feedback, a subtractor unit 51a subtracts from the position command value Xc to output a position error, which is amplified by a position error amplifier Kp1. An output from the position error amplifier Kp1 is added to the velocity feedforward amount VF in an adder unit 52a thereby giving a velocity command value V1.
The subtractor unit 53a subtracts a velocity v1 obtained by differentiating the position x1 in a differentiator 56a from the velocity command value V1 to output a velocity error, which is then generally subjected to proportional integral amplification in a velocity error amplifier Gv1. An output from the velocity error amplifier Gv1 is added to the acceleration/deceleration torque feedforward amount τF1 in an adder unit 57a to be outputted from the first shaft position control unit 100a. 
The second shaft position control unit 100b will not be described here as the inside structure and structural elements thereof are the same as those of the first shaft position control unit 100a. Note that in the tandem control state, the first shaft position control unit 100a and the second shaft position control unit 100b are given a common position command value X from an upper-level device (not shown), and the position command value Xc subjected to acceleration/deceleration processing needs to be common, which means that operations of the acceleration/deceleration processing unit 50a and of the acceleration/deceleration processing unit 50b are the same.
The subtractor unit 58 subtracts the velocity v2 of the drive shaft 2 from the velocity v1 of the drive shaft 1 to output a velocity difference (hereinafter referred to as a deflection velocity). The deflection velocity is amplified by Gd times by an amplifier Gd thereby giving a torque feedback τb. The torque feedback τp is then subtracted from the torque command value, or an output from the first shaft position control unit 100a, in the subtractor unit 59 thereby giving a torque command value τ1 relative to the drive shaft 1 of the position control device 300. In addition, the torque feedback τp is added to the torque command value, or an output from the second shaft position control unit 100b, in an adder unit 60 thereby giving a torque command value τ2 relative to the drive shaft 2 of the position control device 300.
With this structure, the torque command value is corrected so as to reduce generation of deflection. In this case, the torque feedback τp has an effect of reducing vibration due to torque interference between the drive shaft 1 and the drive shaft 2. While I1=0.3 [kg·m2], I2=0.1 [kg·m2], and K=50·103 [Nm/rad] are selected as a target plant condition, preferable control condition (Kp*, Gv*, ATF*: *=1 or 2) is set for this target plant, and a disturbance torque τdis shown in FIGS. 8 and 9 is given in a stepwise manner, and a disturbance response is simulated for every period TS=0.1 [ms]. The result of simulation is shown in FIGS. 11 and 12.
The upper graph in FIG. 11 shows the disturbance torque τdis. With stepwise disturbance of +100 [Nm] given at the time of 10 ms, deflection results between the drive shaft 1 and the drive shaft 2 shown in FIG. 8. The lower graph in FIG. 11 shows the velocity v1 of the drive shaft 1 generated due to stepwise disturbance. The upper graph in FIG. 12 shows the velocity v2 of the drive shaft 2, and the lower graph shows the position error Diff1 (=Xc−x1) of the drive shaft 1.
It is known from FIGS. 11 and 12 that a vibration characteristic of disturbance response significantly varies depending on the amplification rate Gd of the amplifier Gd. With Gd=0→Gd=70 set, torque feedback τp is generated, and damping characteristic is improved. However, with Gd=70→Gd=200 set, damping characteristic becomes excessive, which deteriorates response characteristic. This means that there is a preferable amplification rate Gd, depending on a target plant condition.
Note here that by including deflection velocity detection between control shafts and torque feedback control by the amplifier Gd (hereinafter additionally using the term of torque compensation control) in the equations of motion, namely, the expressions (1), (2), and (3), transmission characteristic of the deflection torque τr relative to the disturbance torque τdis is expressed as an expression (4).
                    [                  Expression          ⁢                                          ⁢          3                ]                                                                                                                τ                r                            ⁡                              (                s                )                                                                    τ                dis                            ⁡                              (                s                )                                              =                      -                                          ω                p                2                                                              s                  2                                +                                                                            G                      d                                        ⁡                                          (                                                                        1                                                      I                            1                                                                          +                                                  1                                                      I                            2                                                                                              )                                                        ⁢                  s                                +                                  ω                  p                  2                                                                    ;                              ω            p                    =                                    K              ⁡                              (                                                      1                                          I                      1                                                        +                                      1                                          I                      2                                                                      )                                                                        (        4        )            
As this characteristic is of a secondary lag system, vibration characteristic can be expressed as an expression (5), using an attenuation coefficient ζ.
                    [                  Expression          ⁢                                          ⁢          4                ]                                                            ζ        =                                            G              d                                      2              ⁢              K                                ⁢                      ω            p                                              (        5        )            
That is, in order to achieve both an appropriate damping characteristic and a response characteristic, it is necessary to select the amplification rate Gd so as to achieve the attenuation coefficient ζ=0.5 to 0.8, depending on the target plant condition.
The conventional position control device shown in FIG. 10 is premised on a structure capable of simultaneous detection of the velocity v1 of the drive shaft 1 and the velocity v2 of the drive shaft 2 and real time calculation of the deflection velocity. However, according to a typical position control device, it is often a case that the first shaft position control unit and the second shaft position control unit are independently formed, and in such a structure, it is not possible on one control shaft side to simultaneously detect on a real time basis the velocity of its own shaft and that of the other shaft. Therefore, it is not possible to execute torque compensation control using a deflection velocity between control shafts relative to a target plant in the tandem control state.
Further, in the case where the condition of a target plant in the tandem control state is constant or varies only slightly, such as is in tandem driving of a feed shaft of a machine tool, it is possible to select an amplification rate Gd in advance while checking vibration characteristic. However, in a case where a workpiece is held on both end portions thereof by two respective main shafts positioned on the opposite sides relative to each other and subjected to turning processing, the workpiece is shifted from being in an independent control state to the tandem control state in which torque interference is caused between shafts, shown in FIG. 8, at the time when one main shaft placed on the opposite side relative to the other main shaft holding one end portion of the workpiece grasps the other end portion of the workpiece.
In such an operation, the target plant condition (inertial moments I1, I2 of the respective shafts and the rigidity K in the tandem control state) varies significantly depending on the material or shape of the workpiece. Therefore, even though the amplification rate Gd is selected in advance, the attenuation coefficient ζ resultantly deviates significantly from an appropriate value once the workpiece is changed to another, and accordingly it is not possible to achieve tandem control with preferable damping characteristic and response characteristic.
In view of the above, the present invention aims to provide a position control device appropriate for tandem driving and capable of promptly achieving torque compensation control with preferable attenuation characteristic even in a position control device including independently formed first and second shaft position control units and incapable of torque compensation control using a deflection velocity between control shafts or in an operation in which the independent control state and the tandem control state are repetitively switched and the target plant condition thus varies significantly.